3 Answers
3

There is one rule that is generally true for all deep sky objects (nebulae, stars, galaxies,...): Aperture matters!

For solar system objects, aperture is not that important.

The second most important thing is: What size are the objects you want to look at: Small objects need long focal lengths and high magnifications, large objects need short aperture for low magnifications.

With 400mm you could watch objects like:

Andromeda galaxy core

Orion nebula, other large emission or reflective nebulae (e.g. Pleiades)

large star clusters

low magnification lunar observations

With 900mm you could watch objects like

Planets (Saturn, Jupiter, Mars, ...)

high magnification lunar observations

planetary nebulae (e.g. ring nebula)

Note that 60 and 70mm aperture are still very small for telescopes! The aperture influences two things:

Light sensitvity: The larger the aperture, the more light you can collect. Very important if you live in a city!

Maximum resolution: Rule of thumb is that you can do aperture in mm times two as maximum magnification. I.e. for 60mm a 120x magnification is the absolute maximum which still is feasible.

The magnification is created by the eyepiece. E.g. when you have a 400mm focal length telescope and use a 10mm eyepiece, you get 400mm/10mm = 40x magnification.

Note: the shorter the eyepiece focal length, the more difficult it is to build. Good 5mm eyepieces can cost 100 USD and up. I personally started with a 750mm Newtonian with 150mm aperture and 25mm and 10mm eyepieces. That's a good allrounder, even though planets will appear rather small with the 10mm eyepiece. But you can later invest more money in good eyepieces, which you can re-use on better telescopes which you may buy later on.

Edit: One more thing -- the telescope mount is equally important as are the eyepieces and the telescope itself. A mount that fits the telescope easily is as expensive as the optical tube assembly itself. Hence many beginners start out with a Dobson telescope, which uses a very, very simple yet sturdy mount.

It depends on what you plan on using it for. For dimmer, deep sky objects you should be concerned with the balance of aperture and focal length (with a focus on aperture for light gathering power). For brighter objects, like the Moon or the solar system planets you probably don't need much light gathering power (aperture) so a long focal length for imaging finer details should be ok. Just a reminder though: never look at the Sun without proper filters, you'll go blind instantly, and looking at the Moon without filters will wreck your night vision and may hurt at first.

+1 on the Moon tip. I once accidentally pointed my scope at part of the moon and had a shadowy streak on my vision for some days. Good neutrally grey polarization filters that can be tuned from 50%-95% absorption can be had for around 50 EUR (for 1.25" eyepieces).
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ArneOct 10 '13 at 7:00

Right or wrong, myself as a beginner I started my decision based on aperture to get the appropriate light gathering ability for my area. At that point focal length became pretty much a non-issue. My choices were down to 2, and 1 of them would have been long enough that getting it outside easily would have been difficult.

Yeah, I'm sorry, I really have no idea what you guys are looking for. A question was asked on what one should do, I answered it, I thought that's what this site was for?
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Brian KnoblauchOct 16 '13 at 19:37